Two-stage chlorine bleaching process with filtrate recirculation

ABSTRACT

A method of bleaching chemical pulp, comprising two bleaching stages (D 0  and, respectively, D 1 ) with chlorine dioxide as dominating bleaching chemical and at least one intermediate alkaline bleaching stage (E). The ingoing pulp is dewatered to a concentration of 25-40% before the first chlorine dioxide stage (D 0 ) and to 10-40% after the second chlorine dioxide stage (D 1 ). Filtrate from the dewatering after the second chlorine dioxide stage (D 1 ) is re-cycled and utilized for controlling the pulp concentration to 8-15% of the dewatered pulp to the first chlorine dioxide stage (D 0 ).

[0001] This invention relates to a method of bleaching lignocellulosicfibrous material in the form of chemical pulp. The invention comprisesmore specifically two bleaching stages with chlorine dioxide asdominating bleaching chemical and at least one intermediate alkalinebleaching stage.

[0002] Bleaching of chemical pulp is carried out in a sequence, whichcomprises several bleaching stages with different bleaching chemicals. Ausual bleaching sequence comprises a first chlorine dioxide stage (D₀)followed by an alkaline bleaching stage (E) and a second chlorinedioxide stage (D₁). The chlorine dioxide stages normally are carried outonly with chlorine dioxide, but alternatively the chlorine dioxide canbe completed with other chemicals, for example ozone. The chemicals areadded in different phases of the bleaching stage. The alkaline bleachingstage normally is an extraction stage with addition of oxygen gas andperoxide (EOP). The alkaline stage alternatively can be completed withan ozone phase, which then is carried out at acid conditions.

[0003] The bleaching of pulp has the object to increase theISO-brightness of the pulp, which implies that impurities anddiscolouring matter are solved out of the pulp and removed bywashing/dewatering before and after the bleaching stages by means offilters or presses. The filtrate from the dewatering can to a largeextent be re-cycled and be used for washing and dilution in previousstages of the process in order thereby to minimize the emissions ofsolved substances. For environmental reasons a low effluent volume isdesired, but the reflux of the filtrate causes a build-up of thesubstance content in the bleaching stages, with the resulting risk ofincreased chemical consumption.

[0004] In bleaching sequences of the aforedescribed and the chlorinedioxide stages are carried out with a pulp of mean concentration(8-15%). As it is desired prior to the first chlorine dioxide stage (D₀)to the greatest possible extent to remove solved substances fromprevious treatment stages, the pulp is dewatered by pressing to highconcentration (25-40%). It is, therefore, necessary by dilution to lowerthe pulp concentration before the pulp is introduced into the bleachingstage. This dilution often is carried out with clean water, but by usinginstead the filtrate from the washing/dewatering directly after thebleaching stage (D₀) the filtrate flow, and thereby also the totaleffluent volume, from the process could be reduced.

[0005] This recycling of filtrate, however, has the disadvantage ofdeteriorating the ISO-brightness of the pulp. In order to compensate forthis, increased chemical addition is required, which involves highercosts.

[0006] The present invention solves the aforesaid problems by a moreadvantageous re-cycling of the filtrate in the process. Thereby anunfavourable build-up of the substance content in the bleaching processis avoided, and the ISO-brightness of the pulp is improved.

[0007] The characterizing features of the invention are apparent fromthe attached claims.

[0008] The invention, thus, relates to a method of bleaching chemicalpulp, comprising two bleaching stages with chlorine dioxide asdominating bleaching chemical and at least one intermediate alkalinebleaching stage. The chlorine dioxide stages (D₀) and, respectively, D₁)can be pure chlorine dioxide stages or bleaching stages where chlorinedioxide is completed with some other bleaching chemical, for exampleozone, in different phases. The chlorine dioxide bleaching is carriedout with pulp of mean concentration, 8-15%. Between the chlorine dioxidestages an alkaline bleaching (E) is carried out, which can be combinedwith the addition of oxygen gas (O) and/or hydroperoxide (P), so-called(EOP)-stage, or ozone (Z(EO)).

[0009] After the bleaching stages the pulp is washed by dewatering, atwhich liquid (filtrate) is removed by means of filters or presses. Thedewatering can be carried out with or without the supply of washingliquid. The pressing suitably takes place in so-called roller presses,where the pulp is pressed between two rollers so that the pulpconcentration is increased to 25-40%. At dewatering by means of filters,however, a pulp concentration of only 10-20% is obtained. After thedewatering the pulp is moved to subsequent treatment stages, where thepulp concentration by liquid addition is adjusted to a level suitablefor treatment in this stage.

[0010] At the method according to the invention, filtrate is re-cycledfrom the dewatering after the second chlorine dioxide stage (D₁) andutilized for controlling the pulp concentration of the pulp dewatered to25-40% before the first chlorine dioxide stage (D₀). The pulp, thus, isdiluted by this filtrate from 25-40% to the concentration suitable inthis stage (D₀), i.e. 8-15%.

[0011] By utilizing in this way the filtrate from D₁, an unfavourablebuild-up of the substance content in D₀ can be avoided. It was,moreover, surprisingly found that the addition of filtrate from D₁results in a lowering of the kappa number and an increase of theISO-brightness of the pulp.

[0012] The invention is described in greater detail in the following,with reference to the accompanying drawing showing an embodiment of aflow sheet for a bleaching method according to the invention.

[0013] Chemical pulp, which can be oxygen gas delignified, is moved viaa first roller press 1 to a first chlorine dioxide stage D₀. The pulp isthereafter via a second roller press 2 moved to an alkaline bleachingstage E and further via a third roller press 3 to a second chlorinedioxide stage D₁ and from there to a fourth roller press 4, from wherethe pulp is moved to a subsequent treatment stage. The second, third andfourth roller press 2, 3, 4 can possibly be replaced by filters. This,however, has the consequence, that the effluent volume increases.

[0014] Before the bleaching step in question, necessary chemicals areadmixed in a conventional manner. To the E-stage suitably also oxygengas and peroxide are added. The necessary amount of fresh water suitablyis supplied to the third roller press and after the second chlorinedioxide stage D₁, see Figure. The filtrate, which is removed as effluentfrom the process, suitably is taken out from the second and third rollerpress 2 and, respectively, 3. The filtrate from the first roller press 1is moved-backward to previous process stages. A small portion of thefiltrates from every roller press 1-4 is recycled in a short circulationto the pulp before the respective press, for example as shown in theFigure. From the fourth roller press 4 the main part of the filtrate isrecycled directly to the pulp in a position between the first rollerpress 1 and the first chlorine dioxide stage D₀.

[0015] In the first, roller press 1 the pulp is dewatered to aconcentration of 25-40%, in order as much as possible to reduce thecontent of undesired substances in the pulp before the D₀-stage.Thereafter the filtrate from the fourth roller press 4 is supplied foradjusting the pulp concentration to the level suitable for the D₀-stage,i.e. 8-15%. After the D₀-stage the pulp is dewatered, and the main partof the filtrate is removed from the process. The concentration of thepulp is controlled again with re-circulated filtrate from the subsequentroller press 3 to a concentration suitable for the E-stage where alsooxygen gas and peroxide are added (EOP). Thereafter the pulp isdewatered and, after control to the desired concentration (8-15%), movedto the D₁-stage. The main part of the filtrate from this dewatering 3 isremoved from the process. The D₁-stage can be a single chlorine dioxidestage or alternatively be carried out with chlorine dioxide in twophases with an intermediate neutralization. After the D₁-stage the pulpis again dewatered and advanced to subsequent treatment stages, whilethe main part of the filtrate is re-cycled to the pulp before theD₀-stage.

[0016] The invention, thus, implies that the bleaching process can bemade more effective in that added bleaching chemicals are utilizedeffectively, at the same time as the effluent volume can be kept low,which is favourable from an environmental point of view.

[0017] In the following example a comparison is made between the methodaccording to the invention and known state of art, whereby thesurprising technical effect becomes apparent.

EXAMPLE

[0018] An oxygen gas delignified coniferous wood pulp with kappa number10.1 was pressed to a pulp concentration of 30% and bleached in achlorine dioxide stage and subsequent alkaline stage according to thesequence D(EO). Before the addition of chlorine dioxide to the D-stage,a dilution of the pulp to 10% was made. For the dilution were used cleanwater, filtrate from a D₀-stage and, respectively, filtrate from aD₁-stage. The result appears from the Table below. TABLE Chlorinedioxide bleaching of oxygen gas deligni- fied pulp with kappa number10.1, ISO-brightness 43% and viscosity 915 mL/g. The kappa factor in thebleaching stage was 1.8 kg active chlorine/ kappa number. Final pH2.3-2.5. After D(EO) Viscosity Dilution with: Kappa numberISO-brightness ml/g Clean water 2.7 71.6 887 D_(o)-filtrate 3.1 68.8 887D_(l)-filtrate 2.5 72.0 894

[0019] The result shows an increase of the kappa number from 2.7 to 3.1after D(EO) when the dilution was made with D₀-filtrate instead of cleanwater. When D₁-filtrate was used, the effect was the opposite one. Thekappa number was lowered after D(EO) from 2.7 to 2.5.

[0020] The invention, of course, is not restricted to the embodimentshown, but can be varied within the scope of the attached claims.

1. A method of bleaching chemical pulp, comprising two bleaching stages(D₀ and, respectively, D₁) with chlorine dioxide as dominating bleachingchemical and at least one intermediate alkaline bleaching stage (E),where the pulp is dewatered to a concentration of 25-40% before thefirst chlorine dioxide stage (D₀) and to 10-40% after the secondchlorine dioxide stage (D₁), characterized in that filtrate from thedewatering after the second chlorine dioxide stage (D₁) is re-cycled andutilized for controlling the pulp concentration to 8-15% of thedewatered pulp to the first chlorine dioxide stage (D₀).
 2. A method asdefined in claim 1, characterized in that the dewatering of the pulpbefore the first stage (D₀) and after the second stage (D₁) is carriedout by roller presses (1, 4).
 3. A method as defined in anyone of thepreceding claims, characterized in that the bleaching in theintermediate stage (E) is carried out as an extraction stage with theaddition of oxygen gas and/or hydroperoxide.
 4. A method as defined inanyone of the preceding claims, characterized in that the dewateringbefore and after the intermediate stage (E) is carried out by rollerpresses (2, 3).